This invention relates to the constituent members of a semiconductor element-manufacturing apparatus such as a silicon single crystal-pulling crucible, heater, process tube and susceptor and a reaction furnace for making said constituent members.
Constuent members of a semiconductor element-manufacturing apparatus are generally known to be prepared from, for example, carbon or quartz glass. In this case, said constituent members have the surface coated, if necessary, with a silicon carbide layer by the chemical vapor deposition (CVD) method. The formation of said silicon carbide layer is intended to prevent a semiconductor material from being contaminated by undesired impurities released from a carbon or quartz glass substrate.
However, a silicon carbide layer produced by the conventional CVD method is formed, as shown in the microscopic photographs of FIGS. 1 to 3, of an agglomeration of fine crystals of silicon carbide (a scale given in the photographs represents 80 microns), which represent a low crystallinity of silicon carbide. Therefore, the silicon carbide layer prepared by the known CVD method has the serious drawbacks that the impurities of the substrate tend to pass through the boundary of silicon carbide particles, and consequently, unless the silicon carbide layer is made considerably thick, it is impossible to prevent a semiconductor material from being contaminated by the aforesaid impurities.
The carbon constituting the substrate of the respective constituent members of the semiconductor element-manufacturing apparatus has a thermal expansion coefficient of 2.5 to 5.5.times.10.sup.-6 /.degree.C. On the other hand, a silicon carbide to be formed on said substrate has a thermal expansion coefficient of 4.2.times.10.sup.-6 /.degree.C. Even when produced by the same process, the carbon substrate indicates considerably wide variations in properties. For example, the thermal expansion coefficient of the carbon substrate generally shows changes of about .+-.10%. Therefore, it is practically impossible to establish coincidence between the thermal characteristics of the carbon substrate and those of the silicon carbide layer. In the case of, for example, a crucible, heater, process tube and susceptor which are repeatedly subjected to heating and cooling, a difference between the extent of thermal expansion of the carbon substrate and that of the silicon carbide layer readily leads to the occurrence of cracks in the silicon carbide layer, particularly when said silicon carbide layer is made considerably thick as 500 microns in order to suppress the permeation of impurities contained in the substrate through the silicon carbide layer. As a result, the effect of suppressing said permeation of impurities can not be realized at all. The growth of cracks in the silicon carbide layer during the use of the constituent members which results from a difference between the thermal characteristics of the carbon substrate and those of the silicon carbide layer is another serious drawback directly related to the contamination of a semiconductor material.
In consideration of the above-mentioned circumstances, the Japanese patent publication No. 1003 (1972) set forth the constituent members such as crucible of a semiconductor element-manufacturing apparatus in which the substrate and a layer mounted thereon were prepared from the same material to ensure coincidence between the thermal characteristics of both substrate and mounted layer. Said Japanese patent publication was intended to suppress the permeation or release of impurities in a substrate material by thermally depositing a layer of thermally decomposable graphite on the surface of a porous graplite substrate. However, said technique had the drawbacks that a layer of thermally decomposable graphite was thermally deposited on the surface of a porous graphite substrate with the axis A of the layer of said thermally decomposable graphite set parallel with the surface of the porous graphite substrate; the thermally deposited layer of said thermally decomposable graphite indicated too great an anisotropy to allow for the repetitive use of the member produced; particularly the edge portion of the thermally deposited layer of said thermally decomposable graphite began to peel even in the initial stage of application of the member, thus rendering the member substantially inapplicable; and the layer of the thermally decomposable grappite which was considerably soft was ready to be mechanically damaged, giving rise to the occurrence of pinholes.
The Japanese patent publication No. 26,597 (1973) proposed an attempt to mechanically improve the adhesivity of a silicon carbide layer to a carbon substrate. The proposed method comprised the steps of letting silicon gas flow over a carbon substrate to effect reaction between the carbon substrate and silicon, thereby forming an intermediate layer of silicon carbide (SiC) prominently adhesive to the carbon substrate; and pouring a silicon-containing gas and a carbon-containing gas, thereby forming a silicon carbide layer by the customary CVD process. However, the method of the Japanese patent publication had the drawbacks that silicon immediately reacted with carbon; consequently unless a silicon gas was let to flow over a carbon substrate uniformly and quickly, an ununiform intermediate layer of silicon resulted, presenting difficulties in effecting the uniform thermal deposition of a silicon carbide layer in the succeeding step. Therefore, the method of the above-mentioned Japanese patent publication was accompanied with rather harmful effect and failed to be put to practical use.
To date, various studies have been made on the formation of a silicon carbide layer. From the point of view that greatest importance is attached to the high purity of silicon carbide when the constituent members of a semiconductor element-manufacturing apparatus are produced, the widely accepted method of forming a silicon carbide layer includes the CVD process using starting materials of high purity, and, above all, the process which involves the following reaction systems: EQU SiCl.sub.4 +[H.C]+H.sub.2 (wherein, H.C. denotes hydrocarbons) (1)
or EQU CH.sub.3 SiCl.sub.3 +H.sub.2 ( 2)
A silicon carbide layer itself thermally deposited by either of the above-mentioned processes indeed has a good purity. However, such silicon carbide layer is not yet freed of the serious drawbacks that impurities contained in a carbon substrate readily tend to permeate the silicon carbide layer on the substrate of the member; and cracks readily take place in said silicon carbide layer because of a difference between the thermal characteristics of the carbon substrate and those of the silicon carbide layer.
The most important reason why it is impossible, as previously described, to ensure coincidence between the thermal characteristics of a carbon substrate and those of a silicon carbide layer is that a starting material of carbon generally has various types and its properties are gradually shifted without definite and noticeable transistion point; and even when the same manufacturing method is applied, it is extremely difficult to produce carbon substrates having the same properties with high reproducibility. With respect to, therefore, a carbon substrate coated with a silicon carbide layer, the conventional process comprises strictly sellecting only those carbon substrates which have preferred thermal characteristics from among a large number of produced lots. To date, therefore, a carbon substrate has been manufactured with an extremely low yield, a factor of deteriorating the economic phase of producing a carbon substrate.